1. Field of the Invention
The present disclosure relates to a fixing apparatus capable of fixing a toner image on a sheet. The fixing apparatus can be used, for example, in an image forming apparatus such as a copying machine, a printer, a facsimile (FAX), and a multifunction peripheral including a plurality of such functions.
2. Description of the Related Art
An image forming apparatus using an electrophotographic method conventionally includes a fixing apparatus which fixes a toner image formed on a recording material (sheet) at a nip portion between two fixing members (first and second fixing rotatable members). A lot of fusibility-improved toners have recently been developed. With improved fusibility, toner can be uniformly and favorably melted by a fixing apparatus. Accordingly, the fixed toner layer is more uniformly and smoothly formed, so that image glossiness can be improved.
As a result, an image having higher glossiness and higher image quality than heretofore can be formed on a high gloss recording material such as coated paper.
As the fixing apparatus repeats fixing processing, the fixing members tend to be deteriorated in surface properties by edge portions of the recording material (both ends in a direction orthogonal to a conveyance direction of the recording material) as compared to other areas. More specifically, the areas touched by the edge portions of the recording material tend to be roughened in the surface as compared to the other areas. Such uneven surface properties of the fixing members may markedly appear on the fixed image as uneven glossiness of the image.
An apparatus discussed in Japanese Patent Application Laid-Open No. 2008-040363 includes a roughening roller (rubbing rotatable member) which rubs the surface of a fixing member. More specifically, the roughening roller rubs the fixing member to make a deteriorated state (surface roughness) of areas touched by edge portions of recording materials less noticeable as compared to the other areas.
On the other hand, if the fixing member is an endless fixing belt, a configuration that causes the fixing belt to reciprocate in a longitudinal direction within a predetermined range is employed in order to prevent the fixing belt from being longitudinally deviation and broken. A configuration that causes the fixing member to reciprocate in the longitudinal direction is also known to reduce the above-described deterioration caused by the contact with the edge portions of the recording material.
Employing the configuration that causes the fixing member to reciprocate longitudinally may increase the range of deterioration of the fixing member by the edge portions of the recording material. Therefore, a width (a longitudinal length) of the roughening roller needs to be increased accordingly.
The inventor has found that the following issue may occur if the width of the roughening roller is set to coincide with a range where the fixing member exists as the fixing member reciprocates.
The issue will be described with reference to FIGS. 19A and 19B. FIG. 19A illustrates a relationship between a contact pressure of a roughening roller with a fixing member and a surface roughness of the fixing member. To maintain the surface roughness of the fixing member within a target range “a” of the surface roughness, at least a contact pressure “b” is needed. The roughening roller is configured to press both longitudinal direction ends of its shaft portions toward the fixing member. Thus, the increased width of the roughening roller causes more than a negligible level of warpage of the roughening roller. As a result, a pressure drop occurs within the range for the roughening roller to rub, or near the center of the longitudinal direction in particular.
FIG. 19B illustrates a relationship between a position in the longitudinal direction and the contact pressure in the rubbing area of the fixing member by the roughening roller. If the contact pressure “b” is secured near the center of the longitudinal direction where the contact pressure is lowest, the contact pressure becomes excessively high at both ends of the rubbing area of the fixing member. Such an excessive contact pressure applies an excessive load to the corresponding areas of the fixing member. Repetition of the rubbing processing may cause cracking or creases in the fixing member.